1. Field of the Invention
The present invention relates to a method and apparatus for optimally recording data on an optical recording medium by adjusting the recording beam power depending upon the state of the recording layer of the optical recording medium.
2. Description of the Related Art
FIG. 1 depicts an optical recording/reproducing apparatus for recording/reproducing data on/from an optical recording medium, comprising a recording signal processing unit 20a for converting input digital data into recording-formatted data after adding an error correction code (ECC), a channel bit encoder 30 for reconverting the recording-formatted data into a bit stream, an optical driver 31 for creating an optical drive signal in response to an input signal, an optical pickup 11 for recording the input bit stream on a recording medium 10 using the optical drive signal or for retrieving recorded data from the recording medium 10, an R/F unit 40 for equalizing and shaping the signal reproduced by the optical pickup 11, a drive unit 50 for driving the optical pickup 11 and a spindle motor M, a servo unit 60 for controlling the drive unit 50 using the rotation speed of the recording medium 10 and tracking and focus error signals, a reproduced signal processing unit 20b for retrieving original data form the binary data outputted from the R/F unit 40, and a microcomputer 70 for controlling the overall recording/reproducing process and the recording beam power.
Before recording data on the recording medium 10, the optical recording/reproducing apparatus performs optimal power calibration (OPC) in the test area of the recording medium 10 to obtain optimal recording beam power.
Upon completion of the optimal power calibration (OPC), the microcomputer 70 controls the optical driver 31 so that input data can be recorded on the recording medium 10 with the obtained optimal recording beam power.
The characteristic of the recording layer, however, changes from inner radius to outer radius. Therefore, although data is recorded with the obtained optimal beam power, the quality of reproduced signal may not be preserved at the outer radius.
As shown in FIGS. 2 and 3, the variation of the recording layer characteristic results from the varying thickness of the recording layer, which is caused by a disk manufacturing process called the spin coating. Generally, the manufacture of optical disks needs the spin coating that creates the recording layer of an optical disk by injecting material for recording layer between the reflective layer and disk substrate while rotating the disk. Therefore, uneven spread of the recording material results in the thickness of the recording layer varying with the disk radius. Owing to the centrifugal force caused by the disk rotation, the thickness of the created recording layer tends to increase from inner radius to outer radius.
Unless the thickness of the recording layer is uniform over the disk, the response of the recording layer to applied recording laser beam also differs with the thickness. As the thickness of the recording layer increases, the extent to which the recording layer is burnt by the recording beam decreases and thus the reflection ratio of recorded marks increases. As a result, the reflection ratio of recorded marks is proportional to the thickness of the recording layer. FIG. 2 shows the trend of reflection ratio of recorded marks over the radius.
The aforementioned optimal power calibration (OPC) is intended to obtain optimal writing power that guarantees the recommended asymmetric ratio xcex2 of reproduced signals. It is likely, however, that the recommended asymmetric ratio xcex2 is unattainable at the outer radius due to the variation of the reflection ratio of marks and therefore the quality of reproduced signal deteriorates at the outer radius.
To solve this problem, the microcomputer 70 continues to control the writing power by measuring the level of the beam reflected in response to recording beam so that the level of the reflected beam remains constant at the test area and data area of the disk. The reflected beam is measured at the point where the level is stabilized sufficiently, as shown in FIG. 3. The process is called the running optimal power control (ROPC).
The ROPC provides high-quality reproduced signals since the reflection level of recorded marks (hereinafter referred to as B-Level 0) is maintained constant over all tracks without regard to the radius, as shown in FIG. 4.
Unlike the beam reflected by marks, the laser beam level reflected by spaces to which recording laser beam has not been applied is not maintained constant by the ROPC (running optimal power control) because the spaces are still subject to the recording layer thickness variation. Since the reflection ratio of spaces decreases as the recording layer thickness increases, the beam level reflected by spaces decreases from inner radius to outer radius. Considering the fact that the beam level reflected by marks remains constant, the asymmetric ratio xcex2 decreases from inner radius to outer radius, as shown in FIG. 5.
The variation of the asymmetric ratio xcex2 leads to the jitter increase of reproduced signals and this in turn increases the likelihood of data error occurrence. Besides, if the ROPC process adjusts the writing power to maintain the reflection level in the area with scratches or fingerprints, the reflection ratio in space intervals relatively decreases and therefore the value of xcex2 becomes lower abruptly in such area. This also increases signal jitter and data errors.
It is an object of the present invention to provide an apparatus and method for controlling optimal writing power for an optical recording medium that maintains a required asymmetric ratio despite recording layer thickness variation at inner and outer radius.
It is another object of the present invention to provide an apparatus and method for controlling optimal writing power for optical recording media that maintains a required asymmetric ratio even in the defect area of the optical recording medium.
The method for controlling optimal writing power for an optical recording medium in accordance with the present invention detects a signal indicative of the thickness variation of the recording layer of the recording medium and controls the writing power to compensate for the characteristic variation of recorded marks due to the thickness variation of the recording layer, based upon the detected signal.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent those skilled in the art from this detailed description.